Which of the following gases are responsible for acid rain? (a) Nitrogen (b)…

Acid rain forms when gaseous pollutants react with water vapour and oxygen in the atmosphere to produce acidic compounds that fall as precipitation. The two major gas groups responsible are sulphur dioxide (SO2) and oxides of nitrogen (NOx, such as nitric oxide, NO), released mainly from burning fossil fuels, vehicle exhaust, and industrial processes.

Sulphur dioxide reacts with atmospheric moisture and oxygen to form sulphuric acid (H2SO4); nitric oxide oxidises to nitrogen dioxide and then reacts with water to form nitric acid (HNO3). Checking each of the five listed gases against this mechanism:

• (a) Nitrogen and (b) Oxygen together make up about 99% of the atmosphere by volume and are chemically stable -- neither reacts with atmospheric moisture to form an acid.

• (b) Oxygen and (c) Ozone: oxygen supports respiration and combustion, while ozone mainly absorbs ultraviolet radiation in the stratosphere -- neither is an acid-rain precursor.

• (c) Ozone and (d) Sulphur dioxide: sulphur dioxide does form an acid, but ozone does not -- so this pairing still fails the mechanism.

• (d) Sulphur dioxide and (e) Nitric oxide: both react with atmospheric moisture/oxygen to form sulphuric acid and nitric acid respectively -- this pairing satisfies the mechanism.

Hence, sulphur dioxide and nitric oxide -- the pair (d) and (e) -- are the gases responsible for acid rain.